Phenol-formaldehyde (PF) adhesive resins have long been used as thermosetting binders in the preparation of manufactured boards or panels. Numerous attempts have been made to develop suitable substitute adhesives, which utilize wood waste, in order to not only meet growing environmental concerns, but to also reduce the dependence upon petroleum based constituents, the availability and cost of which is increasingly uncertain.
These attempts have included efforts to develop suitable adhesive binders by using PF to cross-link other possible adhesive constituents, including one or more waste products of wood processing. Unfortunately, to data these efforts have met with only limited success.
The range of wood waste products which have been successfully cross-linked is limited, and the methods used to produce these adhesive binders have been slow and laborious, often requiring pretreatment of the waste product constituent. Furthermore, the cure time and adhesion characteristics of these adhesive binders have been such that widespread use or acceptance of these substitutes by the manufactured board industry has not occurred. For example, it is often the case that the adhesive binder must be used under acidic conditions. However, board and panel manufacturers prefer to operate in an alkaline environment, since less wear and tear is encountered by production equipment.
In U.S. Pat. No. 2,786,008 issued on Mar. 19, 1957--Herschler, an adhesive binder consisting of phenol-formaldehyde (PF) and ammonium based spent sulphite liquor (NH.sub.4 SSL) is disclosed. In addition to producing an acidic adhesive which is slow curing, the method disclosed by Herschler is laborious and time-consuming, as it is necessary to make the alkaline PF "acid tolerant" in order to avoid precipitation of the PF resin upon mixing of same with the acidic spent sulphite liquor.
Another example of using PF to cross-link a wood waste product is that disclosed in U.S. Pat. No. 4,113,675 issued on Sep. 12, 1978--Clarke et al., wherein methylolated lignin was cross-linked by an acid catalyzed low molecular weight PF resin. It was observed that kraft lignin was itself not sufficiently reactive to under significant cross-linking with PF, and that it was thus necessary to pre-react the lignin with a methylolating agent, such as formaldehyde, so as to introduce methylol groups to the lignin molecule. Moreover, it was indicated that in order to achieve satisfactory cross-linking on curing, the pH of the binder must be acidic. Finally, the adhesive disclosed in Clarke et al. was found to be relatively slow curing.
In Canadian patent 1,214,293 issued on Nov. 18, 1986--Calve et al., an adhesive consisting of ammonium based spent sulphite liquor (NH.sub.4 SSL) cross-linked by a commercial PF is disclosed. It was found that while PF normally precipitates when in acidic solution with NH.sub.4 SSL, a useful adhesive could be obtained if the NH.sub.4 SSL-PF were maintained in dispersion through rigorous stirring or agitation. Again, acidic conditions, of pH 3 to 7, were required to provide acceptable modulus of rupture (MOR) test results on waferboard and particleboard manufactured with the adhesive.
In U.S. Pat. No. 4,127,544 issued on Nov. 28, 1978--Allan, a process for the partial substitution of ammonium lignosulfonate for phenol in alkaline phenolic-aldehyde resin adhesives is described. However, the NH.sub.4 SSL is first pre-reacted with phenol at a temperature range of 150.degree.-300.degree. C. under autogenous pressure prior to condensation with formaldehyde under alkaline conditions. The reaction at high temperature and pressure is expensive. Also, no wood adhesive bond test data were provided.
In U.S. Pat. No. 4,324,747 issued on Apr. 13, 1982--Sudan et al, a resin where an alkaline phenol-formaldehyde resin is simply mixed with a kraft pulping liquor and use to bond waferboard is disclosed. The phenolic resin is prepared by refluxing phenol and formaldehyde in the presence of zinc acetate or heating at lower temperature in the presence of calcium oxide. No reference to adjustment of molecular weight distribution of the phenolic resin or example of addition of ammonium salt is provided. This type of resin is slow curing and not "pre-cure" resistant.
To be acceptable for industrial use, a new adhesive must meet certain criteria. For example, it must be available as a spray dried powder or stable liquid; be quick setting if it is employed as a core adhesive for thick multi-layer panels where the resin is far from the heating source, or be "pre-cure" resistant if it is employed for bonding a monolayer panel or the face of a multi-layer panel. If the resin cures prematurely while the mat is resting on a hot caul plate before pressing or at first contact with the press platens before sufficient pressure is applied to consolidate the mat, a poor bond will ensue.
In the aforementioned examples and elsewhere, it has not been suggested or contemplated that the PF component of the adhesive be modified or specially constituted to enhance the cross-linking of the wood waste product component of the adhesive.
While a two component or "binary PF" resin, comprised of a mixture of high average molecular weight PF and low average molecular weight PF components, has long been used in the manufacture of board, heretofore, the low molecular weight PF has been added to enhance resin flow with minimum dry-out. Its unique reactivity to produce a quick setting resin when used in combination with wood waste, especially polydisperse lignin, has not been recognized and its use in combination with wood waste product not contemplated.
For example, in U.S. Pat. No. 4,269,949 issued on May 26, 1981--Hickson et al, there is disclosed a binary PF resin comprised of a mixture of high and low molecular weight resins, particularly suitable for use in hardboard applications, owing to the minimum dry-out exhibited by the adhesive.
In U.S. Pat. No. 4,433,120 issued on Feb. 21, 1984--Chiu, it was demonstrated that a liquid binary PF resin having low viscosity and low surface tension could be used for efficient spray application as fine droplets in waferboard manufacture. The liquid binary PF resin has limited pre-cure resistance due to the presence of the slower curing low molecular weight phenolic resin which is however not sufficient for a surface resin. Although the liquid binary PF resin exhibited some pre-cure resistance, this resin is relatively expensive to produce, and displayed only limited pre-cure resistance. It is also difficult to spray-dry without advancing the resin and reducing its pre-cure resistance.
In a recent study Stephens and Kutscha (Wood and Fiber Science, 19(4), 1987, pp. 353-361) fractionated a phenol-formaldehyde resin into a high and low molecular weight resin by ultrafiltration and compared the adhesive properties of each fraction to the unfractionated PF resin. They found that: "the high molecular weight resin fraction performed nearly as well as the unfractionated resin". The presence of low molecular weight phenol-formaldehyde only slightly improve the characteristic of the PF resin, and is not essential to the process.